An internal combustion engine includes an oil pan that stores oil. The oil is supplied to and lubricates parts requiring lubrication in the engine. After being used for lubrication, the oil is returned to the oil pan.
Patent Document 1 discloses an internal combustion engine, in which some of the oil used for lubricating parts requiring lubrication trickles down along an inner wall surface of the cylinder block and is returned to the oil pan. Such an internal combustion engine has an oil passage that connects the interior of the crankcase and the interior of the cylinder head. The oil that has been used for lubricating parts requiring lubrication in the cylinder head is returned to the crankcase through the oil passage, and some of the oil trickles down along the inner wall surface of the cylinder block and is returned to the oil pan.
An internal combustion engine, in which the oil passage is open at the upper section of the crankcase, has also been proposed. In such an internal combustion engine, some of the oil returned to the crankcase trickles down along the inner wall surface of a skirt of the crankcase and is returned to the oil pan.
In the internal combustion engine configured as described above, the crankshaft rotates at high speed in the crankcase of the cylinder block. As the crankshaft rotates at high-speed, the connecting rods operates at high speed.
Thus, when the direction of movement of operating members such as the crankshaft and the connecting rod intersects the flowing direction of the oil that trickles down along the inner wall surface of the skirt, air flow generated in accordance with the operation of the operating members or the operating members themselves interfere with the oil that trickles down as described above.
This undesirably stirs the oil thereby generating air bubbles in the oil, and increases the rotational resistance of the operating members, which increases the fuel consumption.
Patent Document 1: Japanese Laid-Open Patent Publication No. 8-135423